If that sounds like an awkward pairing, it is. “Seven Psychopaths” is manic and messy (particularly in the first half) and McDonagh — whose previous film was the more centered “In Bruges” — doesn’t yet have the visual command for a sprawling, madcap tale as this.

But it’s also filled with deranged wit and unpredictable genre deconstruction that makes “Seven Psychopaths” if not quite a success, a fascinating mutt of a movie.

Colin Farrell plays Marty, a hard-drinking screenwriter in Los Angeles and a clear stand-in for McDonagh. (The first letters of his last name are pointedly obscured on his scripts, but “McDonagh” is coyly suggested.) He has his movie title — “Seven Psychopaths” — but little else.

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His friend Billy (Sam Rockwell) is his sounding board. But as Marty tries to write, he gets sucked into Billy’s own hijinks. With the help of his older friend Hans (Christopher Walken), Billy kidnaps dogs and then returns them for the reward money. This practice gets them in trouble when they swipe the Shih Tzu of a dog-loving gangster (Woody Harrelson). Bloody bodies quickly accumulate.

That this is the plot isn’t necessarily clear until fairly well into “Seven Psychopaths.”At first, it’s paced by stylish introductions of various psychopaths, some of whom are fictional inventions — like a murderous Vietnamese priest (Long Nguyen) — and some of whom are among the main characters.

It’s an excellent cast: Others include Harry Dean Stanton, Abbie Cornish, Kevin Corrigan, Gabourey Sidibe and a bunny-cradling Tom Waits. But this is Rockwell’s movie. The actor has long specialized in loose cannons (“Moon,” “Snow Angels”) but his Billy may be the most fun yet.

He enthusiastically supports Marty, trying to get him to write, while revealing that he, too, might be a fittingly unhinged character for the script. But even in his darkest moments, he’s gleeful at the movie he’s acting out. Many of McDonagh’s best lines are his. Explaining that he didn’t mean to suggest his girlfriend is carrying a worse venereal disease, he sweetly says, twice: “Honey, I meant like chlamydia or something.”

In urging Marty’s script forward, Billy also pushes along “Seven Psychopaths.” Billy — whose last name, Bickle, evokes Robert De Niro’s Travis — is the movies, themselves: violent, hysterical, lunatic and totally captivating. His suggested cliched vision for Marty’s script (the psychopaths team up for a cemetery shootout) could easily be in theaters any given week. (“Smokin’ Aces” comes to mind.) He is the excited advocate for gunplay, action and, absolutely, a big showdown finale — both in the script and in his life, if there’s a difference.

Marty, though, wants his film to be about “love and peace” and halfway through “Seven Psychopaths,” he contemplates a sudden turn away from the expected plot mechanics. He imagines the characters simply leaving their guns, going to the desert and talking.

Apoplectic, Billy responds: “What are we making, French movies now?”

For a while, this is exactly what “Seven Psychopaths” does and it’s when it finds its footing. McDonagh is best in such Beckett-like limbos heavy with Catholic guilt — the delightfully grim “In Bruges” was essentially set in purgatory, a.k.a. Belgium. The very talented writer-director has often drawn fair comparisons to Quentin Tarantino (both enjoy the chit chat of hit men), but McDonagh’s work has a darker soulfulness, even when meta playfulness like that in “Seven Psychopaths” obscures it.

Also in the desert, Walken’s character — whose slow, deliberate enunciations are like a soothing metronome for film — takes peyote, which is worth the price of admission, alone.

After breaking apart the crime film, McDonagh puts it back together again for a conclusion worthy of the genre. In the end, the movies — in all their insanity — win. The French lose.